The present invention relates to a drive device.
A conventional drive device includes a reduction drive that includes a worm, which is driven by a motor, and a gear, to which rotation of the worm is transmitted. The reduction drive is accommodated in an accommodation portion. The drive device reduces the speed of the rotation produced by the motor with the reduction drive and outputs the rotation to the outside from an output unit, which is coupled to the reduction drive. For example, Japanese Patent No. 5335107 describes a drive device including an accommodation portion that includes an annular support projection. The support projection projects in the same direction as a support shaft that supports a worm wheel, which meshes with a worm. The support projection supports the worm wheel in a slidable manner.
Resin gears are less expensive than metal gears, and helical gears produce less vibration and noise and have stronger teeth than spur gears. Thus, resin helical gears are used in a reduction drive. However, meshed helical gears generate a thrust force. Further, when meshed helical gears reduce the rotation speed of a worm, the helical gear that is close to an output unit in a power transmission path of a reduction drive has a larger diameter than a helical gear that is far from the output unit. This increases the distance between the center of the rotation and the meshed portion. Thus, when, for example, movement of the output unit is constrained while the motor is being driven, a large load is applied to the output unit. This increases the thrust force at the meshed portion of the helical gears and causes the helical gear that is close to the output unit in the power transmission path of the reduction drive to be greatly bent in the axial direction. As a result, the meshing of the helical gears may become insufficient. Further, it is difficult to avoid insufficient meshing of the helical gears with only the support projection that supports the worm wheel, which meshes with the worm, as described in the above document.